This invention relates to pipettes for initially drawing a preselected volume(s) of liquid and subsequently discharging a precise volume(s) of the drawn liquid. More particularly, the invention relates to air-displacement pipettes in which a disposable tip typically contains the drawn liquid, and an air buffer separates the drawn liquid from the piston and cylinder structure typically utilized for drawing and discharging the liquid, so as to prevent contamination of the primary operational elements of the pipette. Specifically, the invention is directed to a multichannel liquid-end assembly for an air-displacement pipette, the liquid-end assembly comprising modular components and further comprising means for facilitating ejection of a spent tip(s).
Various configurations for air-displacement pipettes are known. One known configuration comprises a single barrel, a single piston and cylinder structure within the barrel, a manual or motor-driven linear actuator at one end of the barrel, and a disposable tip at the other end of the barrel. Examples of manually-operated air-displacement pipettes of this type appear in Reed, U S. Pat. No. 3,918,308, and d'Autry, U.S. Pat. No. 3,991,617. Nishi, U.S. Pat. No. 3,915,651, and Magnussen, Jr. et al., U.S. patent application Ser. No. 06/580,587, filed on Feb. 16, 1984, entitled METHODS AND APPARATUS FOR PIPETTING AND/OR TITRATING LIQUIDS USING A HAND HELD SELF-CONTAINED AUTOMATED PIPETTE, and assigned to the same assignee as this application, disclose a motor-driven air-displacement pipette of this type. The referenced manually-operated air-displacement pipettes and the automated pipette disclosed in aforementioned U.S. Ser. No. 06/580,587 further comprise ejection mechanisms typically comprising a manually actuatable sleeve slidable along the exterior surface of the barrel for dislodging a spent tip.
Although the single-channel air-displacement pipettes mentioned above have many uses, in some instances, identical volumes of liquid are sequentially drawn and discharged. The use of a single-channel pipette requires repetitive, time-consuming procedures. Furthermore, in the case of manually-operated pipettes, these repetitive procedures can cause user fatigue and result in imprecise pipetting. Therefore, multichannel configurations for air-displacement pipettes have been developed.
One known multichannel air-displacement pipette is marketed under the registered trademark TITERTEK. The TITERTEK pipette is manufactured by EFLAB OY of Finland and distributed by Flow Laboratories, located in Waltham, Mass. This pipette has a unitary liquid-end assembly comprising a plurality of barrels, a piston and cylinder structure in each barrel, a common actuator connected to each of the pistons, and a tip ejector comprising a thumb-actuatable, spring-returned rake movable along the barrels for dislodging spent tips. One disadvantage of this pipette is that the entire operational mechanism must be replaced if any piston and cylinder is contaminated or worn, since the operational structure is unitary. Another disadvantage is that the user must exert substantial effort when he or she advances the rake against the tips in order to dislodge spent tips from the barrels, since the ejection mechanism does not provide a mechanical advantage.
Another multichannel air-displacement pipette is marketed under the registered trademark CO-STAR, by Co-Star. Inc., located in Cambridge, Mass. As in the case of the TITERTEK pipette, the CO-STAR pipette has a unitary liquid-end assembly with the same disadvantage mentioned above that the entire operational mechanism must be replaced if any piston and cylinder is contaminated or worn. Unlike the TITERTEK pipette, however, the CO-STAR pipette has a tip ejector comprising a spring-returned rake having a top surface in the form of an inclined plane and associated with a finger-actuatable trigger slidable in a track, which moves the wedge-shaped rake downwardly for dislodging spent tips, so that mechanical advantage is applied. However, the rake tends to tilt and bind, which renders the ejection mechanism difficult to operate.
Additionally, disposable tips typically are supplied in trays containing numerous tips. The tips are positioned in the trays such that the distance between adjacent tips corresponds to the distance between adjacent barrels on multichannel air-displacement pipettes, so that the tips can be stabbed onto the barrels. Unfortunately, the unitary liquid-end assemblies of the TITERTEK and CO-STAR pipettes preclude removal of selected barrels so as to be able to stab only so many tips as are desired to be used. Consequently, if less than all channels are desired to be used, the tips must first be stabbed onto the barrels and, thereafter, selectively manually removed or, alternatively, the trays must be specially sorted so that tips are stabbed only onto selected barrels. This is very inconvenient.
It is desirable that a multichannel air-displacement pipette be provided, which enables only contaminated or worn parts to be replaced in lieu of replacing the entire operational mechanism. It is also desirable that a multichannel air-displacement pipette be provided, which enables selected channels to be used without having to remove tips stabbed onto the pipette or, alternatively, having to rearrange pipette tip trays. Furthermore, it is desirable that spent tips be effectively dislodged without excessive difficulty, so that the user is not reconciled to using his or her fingers to assist removal of tips which can often be contaminated with noxious or injurious liquids.